When Joan Lunden was diagnosed with triple negative breast cancer, or TNBC – a rare and aggressive form of the disease – the former Good Morning America co-host was initially "shell-shocked." But Lunden turned a corner quickly and is focusing on finding strength through the love and support of her family, fans and colleagues.

After finishing her second chemotherapy treatment on June 26, "she was expected to start losing her hair in the coming week," a close family friend tells PEOPLE. "So she took matters into her own hands and shaved her head. She wanted to do it on her own terms, and she looks stunning."

Lunden, 63, first revealed that she was battling breast cancer on June 24 during a sit-down with Robin Roberts on Good Morning America, where she spent 17 years as an anchor.

"When something like this happens, you learn just how important it is to have a support system," she said during the interview.

That support system – which includes husband Jeff Konigsberg and her seven children (two sets of twins, 9 and 11 years old, with Konigsberg, plus three older daughters from a previous marriage, to Michael Krauss) – has helped motivate her tremendously throughout her cancer battle.

"Right away, she went into lioness mode protecting her family," the source says of Lunden's reaction. "She didn't want to upset the kids before summer camp."

And when it comes to her passion for being a health advocate, Lunden isn't planning on letting her diagnosis get in the way. As the family friend recalls, Lunden told her, " 'I'm having 200 women at my health and wellness retreat in August, and I have to be there.' "

While Lunden is aware that she has a challenge ahead of her, the TV host and author has decided to find a purpose in her diagnosis. Lunden, the source says, has vowed to "take this as an opportunity to try and inspire others to protect their health."